Question

In the $NaCl$ (Rock salt) structure, what would be the distance for the nearest cation $\left(N{a}^{+}\right)$ from an anion $\left(C{l}^{-}\right)$ ?
Here $a$ is the edge cell length.

• A. $\frac{a}{4}$
• B. $\frac{a}{2}$
• C. $\frac{a}{\sqrt{2}}$
• D. $a$

Answer 1

The correct option is B $\frac{a}{2}$

In the $NaCl$ (Rock salt) structure,
$C{l}^{-}$ ions form the FCC lattice,while $N{a}^{+}$ ions occupy all the octahedral voids.


For an anion $C{l}^{-}$,
Distance of the nearest cation $N{a}^{+}$ will be $\frac{a}{2}$.
This is the distance from the corner atom $C{l}^{-}$ to the edge atom $N{a}^{+}$
Option (b) is correct.
Number of nearest cations are 6
Nearest anion-anion distance is $\frac{a}{\sqrt{2}}$
This is the distance between face centre and corner atoms.
In a rock salt $NaCl$ structure:
Effective number of $C{l}^{-}$ ions in each unit cell are 4
Effective number of $N{a}^{+}$ ions in each unit cell are 4
Effective total number of $NaCl$ units in a unit cell are 4